150 rhesus monkey alloantisera raised by planned immunization or received from other laboratories were evaluated for lymphocytotoxic antibodies by (1) titration against peripheral blood lymphocytes of selected monkeys; (2) typing of 112 random unrelated monkeys and of members of 30 monkey families by standard microcytotoxicity methods. This resulted in an increase of RhL-A serologically defined antigens which we recognize from 11 to 18 including 7 in the first series and 11 in the second. Quality of sera defining antigens has been greatly improved in each case since (1) frequency of defining sera for each group is very close; (2) X2 of correlation of sera in pairwise comparison of defining sera for each group is very high. One possible third series antigen is being evaluated. Two monkeys have been identified which behave as LD typing cells or homozygotes at the major locus determining MLC stimulation (LD). These are being used to type families and random unrelated populations for the 2 LD antigens. Breeding to produce RhL-A homozygotes and RhL-A identical siblings is producing about 10 live births per year.